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Objective Type Set
Online MCQ Assignment
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1. RC5 encryption uses Right shift and decryption uses Left shift.
a) True
b) False

Answer: b [Reason:] RC5 encryption rounds use left shift operations and the decryption uses right shift operations.

2. The above round is the last step in encryption /decryption in RC5.
a) True
b) False

Answer: b [Reason:] The last round is the last step in encryption but the first step in decryption process.

3. The code bellow is the RC5 encryption pseudo code in C language.
What is the error?
LE0 = A + S;
RE0 = B + S ;
for i = 1 to r do
LEi = ((LEi-1 ⊕ REi-1) >>> REi-1) + S [2xi];
REi = ((REi-1 ⊕ LEi) <<< LEi) + S [2xi+1];

a) The left shift operation (should be right shift)
b) The right shift operation (should be left shift)
c) LEi-1 and REi-1 should be LEi and REi
d) The for loop runs from 1 to r-1 ( not 1 to r)

Answer: [Reason:] Correct code – LE0 = A + S; RE0 = B + S ; for i = 1 to r do LEi = ((LEi-1 ⊕ REi-1) <<<REi-1) + S [2xi]; REi = ((REi-1 ⊕ LEi) <<< LEi) + S [2xi+1];

4. “RC5 uses the Feistel Structure.”
a) True
b) False

Answer: b [Reason:] RC5 does not use the classic Feistel structure.

5. Find the error in the Decryption pseudo code for RC5 –
for i = 1 to r do
RDi-1 = ((RDi – S [2xi+1] >>> LDi ) ⊕ LDi);
LDi-1 = ((LDi – S [2xi] >>> RDi-1) ⊕ RDi-1);
B= RD0 + S ;
A = LD0 – S ;

a) B= RD0 + S ;
b) for i = 1 to r do
c) LDi-1 = ((LDi – S [2xi] >>> RDi-1) ⊕ RDi-1);
d) A = LD0 – S ;

Answer: a [Reason:] B= RD0 + S ; is incorrect as there should be a ‘-’ instead.

6. Which one of the following is not a RC5 mode of operation?
a) RC5 block cipher
b) RC5-Cipher Block Chaining
d) RC5-CipherText Stealing

Answer: c [Reason:] RFC 2040 [BALD96] defines four different modes of operation: 1. RC5 block cipher 2. RC5-CBC 3. RC5-CBC-Pad 4. RC5-CTS.

7. Which RC5 mode will have the ciphertext longer than the plaintext by at most the size of a single RC5 block?
a) RC5 block cipher
b) RC5-Cipher Block Chaining
c) RC5-Cipher Block Chaining Pad
d) RC5-CipherText Stealing

Answer: c [Reason:] The Cipher Block Chaining Pad mode produces a ciphertext output as such.

8. Which RC5 mode of operation is this?
a) RC5 block cipher
b) RC5-Cipher Block Chaining
c) RC5-Cipher Block Chaining Pad
d) RC5-CipherText Stealing

Answer: d [Reason:] This mode of operation is the RC5 CTS mode.

9. Which of these is not a characteristic of block ciphers?
a) Variable key length / block size / number of rounds
b) Mixed operators, data/key dependent rotation
c) Key independent S-boxes
d) More complex key scheduling

Answer: c [Reason:] Features seen in modern block ciphers are: – Variable key length / block size / number of rounds – Mixed operators, data/key dependent rotation – Key dependent S-boxes – More complex key scheduling – Operation of full data in each round – Varying non-linear functions.

10. Which one of the following RC4 algorithm not used in?
a) SSL
b) TLS
c) FTP
d) WEP

Answer: c [Reason:] RC4 is used in Secure Socket Layer, Transport Layer Security and Wired Equivalency Privacy. Not used in File Transfer Protocol.

11. Till when was the RC4 algorithm kept a secret?
a) 1990
b) 1992
c) 1996
d) 1994